The P-40Q was an experimental
project which attempted to produce a really modern fighter out
of the existing P-40. The modifications were in fact so drastic
that there was very little in common with earlier P-40 versions.

Two P-40Ks (serial numbers 42-9987
and 42-45722) and one P-40N (serial number 43-24571) were extensively
modified with revised cooling systems, two-stage superchargers,
and structural changes which markedly altered their appearance.
The project was assigned the designation XP-40Q.

The first XP-40Q was P-40K-10-CU
ser no 42-9987 fitted with a new cooling system, a longer nose,
and a four-bladed propeller. The radiators were moved into an
under-fuselage position, with intakes between the undercarriage
legs.

The most prominent XP-40Q feature,
used on 42-45722 and 43-24571, was the cutting down of the rear
fuselage and the addition of a bubble canopy as on the "XP-40N".
Later the wingtips were clipped. The result was an aircraft
which bore almost no resemblance whatsoever to its parent P-40
line. The V-1710-121 engine was fitted with water injection,
resulting in a power of 1425 hp. Speed increased to 422 mph
at 20,500 feet, making it the fastest of all the P-40s. An altitude
of 20,000 feet could be reached in 4.8 minutes, and service
ceiling was 39,000 feet. Four 0.5-inch machine guns were carried
by the prototypes. Wingspan was 35 feet 3 inches (after clipping),
and length was 35 feet 4 inches (2 feet longer than the P-40N).

The proposed production models
of the P-40Q were to have carried either six 0.50-inch machine
guns or four 20-mm cannon, but the XP-40Q was still inferior
to contemporary production Mustangs and Thunderbolts, and development
was therefore abandoned. Consequently, the production life of
the P-40 ended with the N version.

The second XP-40Q was briefly
used for postwar air racing. Registered NX300B, the second XP-40Q
was an unauthorized starter in the 1947 Thompson Trophy race.
It was in fourth place when it caught fire and had to drop out
of the race.

Sources:

War Planes of the Second World
War, Fighters, Volume Four, William Green, Doubleday, 1964.